Tyrosyl peptides are being isolated from the binding sites of rabbit and mouse antihapten antibodies and of ligand-binding mouse myeloma proteins and their positions in the primary sequences of the heavy and light chains determined. The findings will aid in establishing the role of particular hypervariable sequence regions in specific combining sites. Whether some binding sites contain two or more tyrosyl residues and whether they are from two different regions of the same heavy or light chain or one is from a light chain and one a heavy chain of the same molecule will be determined. Relatively large amounts of antibodies of restricted heterogeneity will be obtained by the use of antigens found to give a restricted response, by the use of preparative isoelectric focusing and as antibodies carrying cross-reactive idiotypes in the antisera of particular strains of mice and possibly a closed rabbit colony. Combinations of these procedures will be used. Binding-site peptides from a specific antibody population common to a particular strain of mice will be sought in several combinations as V-gene markers. The labeling of tyrosyl residues in the combining sites of the antibodies or ligand-binding myeloma protein is through the paired iodination procedure. The significance of this work is that a knowledge of the structure and specificity relationship of many antibodies must be obtained in order to understand binding site specificity. It is also important in establishing the mechanisms of antibody biosynthesis and its genetic control. The sequences of antibodies will permit synthetic production of therapeutically active antibodies such as cytotoxic anti-cancer antibodies.